In forming a wellbore for oil and gas exploration and production, a pilot wellbore is formed. The pilot wellbore may extend about vertically into the surrounding formation and may be cased, uncased, or have portions that are cased and other portions that are uncased. After forming the wellbore, a portion of the wellbore may thereafter be widened. For instance, the wellbore may be widened to accommodate larger sizes of casing. The process for widening the wellbore may include using an underreamer to expand the diameter of the wellbore.
An underreamer includes a cylindrical body and multiple cutter blocks that can move and expand relative to the cylindrical body. The cutter blocks move between a retracted position and an expanded position. In the retracted position, the cutter blocks are folded into corresponding recesses in the cylindrical body. In the retracted position, the cutter blocks and the cylindrical body can be run into the wellbore and lowered to a position where the wellbore is to be widened. When the underreamer is in a desired position in the wellbore, hydraulic fluid can be pumped into a drill string coupled to the underreamer. Through hydraulic pressure, the cutter blocks can be actuated and move radially outward relative to the cylindrical body. The cutter blocks move radially outward to contact the wall around the wellbore. By rotating the underreamer in the wellbore, the cutter blocks can cut into the wall of the wellbore and increase the diameter of the wellbore. The underreamer can also be moved axially while the cutter blocks are in the expanded position to expand the diameter of the wellbore along an axial portion of the wellbore.